getMoonPhase.js

const phases = [{
  phase: "new",
  min: 0,
  max: 1
},
{
  phase: "waxing crescent",
  min: 1,
  max: 6.38264692644
},
{
  phase: "first quarter",
  min: 6.38264692644,
  max: 8.38264692644
}, {
  phase: "waxing gibbous",
  min: 8.38264692644,
  max: 13.76529385288
}, {
  phase: "full",
  min: 13.76529385288,
  max: 15.76529385288
}, {
  phase: "waning gibbous",
  min: 15.76529385288,
  max: 21.14794077932
}, {
  phase: "last quarter",
  min: 21.14794077932,
  max: 23.14794077932
}, {
  phase: "waning crescent",
  min: 23.14794077932,
  max: 28.53058770576
}, {
  phase: "new",
  min: 28.53058770576,
  max: 29.53058770576
}];

function fmod(dividend, divisor) {
  let multiplier = 0;
  while(divisor * multiplier < dividend) {
    ++multiplier;
  }
  --multiplier;
  return dividend - (divisor * multiplier);
}

function getMoonPhase(date) {
  const $unixdate = Math.round(date.getTime() / 1000);
  const $lunardays = 29.53058770576;
  const $lunarsecs = $lunardays * (24 * 60 *60);
  const $new2000 = Math.round((new Date('2000-01-06T18:14:00.000Z')).getTime() / 1000);
  const $totalsecs = $unixdate - $new2000;
  let $currentsecs = fmod($totalsecs, $lunarsecs);
  if ($currentsecs < 0) {
    $currentsecs += $lunarsecs;
  }
  const $currentfrac = $currentsecs / $lunarsecs;
  const $currentdays = $currentfrac * $lunardays;
  const { phase } = phases.filter(each => ($currentdays >= each.min && $currentdays <= each.max))[0];

  return {
    phase,
    age: $currentdays.toFixed(2),
    lunation: ($currentfrac*100).toFixed(2),
  };
}

console.log(getMoonPhase(new Date('2022-02-21T03:00:00.000Z')));